I have been going nuts over this for a long time.
I have followed your steps and it was still not working. Couldn´t believe it, so:
As I always render to viewport, I tried to render to Live Viewer window and indeed it DOES render the background there, but NOT in the viewport.
Also the background object with the octane diffuse material is not showed in the active camera viewport when octane render is paused or not started.
So I need to have
1- backround object with a standard c4D material YES visible on editor/NOT visible to render
2-backround object with a octane diffuse material NOT visible on editor/ YES visible to render
nVidia GTX 1080
Any help will be very wellcomed.